I've slightly revised this paper
http://www.cs.berkeley.edu/~fateman/papers/addformat.pdf
also available via
http://tinyurl.com/44jdf/addformat.pdf
which may be of interest to people who are interested
in inserting a new format for expressions of some
special category into Maxima. It can be done in various
ways, but a really thorough integration into the whole
system is difficult. Fortunately you may need much
less.
If you looked at this paper when it was first posted, July, 2004,
you've seen all the technical material, which has not changed.
for earlier comments, see
http://www.math.utexas.edu/pipermail/maxima/2004/007713.html
RJF